Detrital mineral geochronology and geochemistry
Wind and water are the great enemies of Earth's continents: they constantly seek to destroy and tear down topography, reducing great mountain ranges to literal piles of sand. In fact, those piles of sand (or sandstones) are sometimes the only records we have left of episodes of mountain building and other great geologic events in the deep past. Detrital mineral research seeks to interrogate individual sand grains in order to track them back to [potentially now vanished] sources and to understand the geologic history of unexplored regions.
Most detrital mineral studies involve U-Pb dating of detrital zircons. Zircon ages provide a good baseline on the igneous history of the source region but they tend to miss the metamorphic history of the source region. We also cannot tell based on age alone what kind of rock the zircon originally came out of. However, we can date other U-bearing minerals and we can also look at the trace element compositions of both zircons and these other minerals in order to better understand what kind of lithologies they were originally derived from. To this end, I have set up methods in the laser lab here to simultaneously measure both U-Pb age and key trace element ratios in zircon, monazite, rutile, and titanite by LA-ICP-MS. A pilot study has recently been published in G-Cubed and involves sand from the Merrimack River in New England. The figure below summaries the findings and the individual layers of information that are provided by the different minerals.
Two projects applying these tools to ancient sedimentary systems are currently underway. One study, recently funded by NSF, focuses on Mesoproterozoic Belt-Purcell Supergroup in Montana, Idaho, and adjacent Canada and age equivalent rocks in the SW US. These units represent sedimentary basins that formed when another continent was linked to western North America and they received some of their detritus from this mystery continent. This work seeks to differentiate between various continental reconstruction models that place either North Australia, South Australia, East Antarctica, or another continental mass next to western North America. Undergraduate Aaron Leonard and graduate student Rick Butts have worked on this.
The second project deals with the question of terrane translation during the late Mesozoic/early Cenozoic in western North America, exemplified by the "Baja-BC hypothesis", which proposes that rocks in SW British Columbia slid along transform faults several thousand km north in the Cretaceous-Paleogene. In particular, we are looking at a thin strip of rocks in SW Oregon that may have experienced some of this translation. Undergraduate Ericka Boudreau is working on this.